1. Field of the Invention
This invention relates to a method and apparatus for measuring the resistance offered by insulating members to an impressed voltage tending to produce a leakage current through or on the surface of the members. More particularly the present invention relates to conductively coupling each of a plurality of insulation specimens between a high voltage test potential and individual ones of a plurality of converters having inputs receiving the leakage current and outputs that are selected by multiplex switching and varies in response to the leakage current; the outputs are characterized by a low voltage signal which is interrogated by analog switching without introducing erroneous electrical influences to the electrical signals used for determining the resistance measurement of the insulation specimen.
2. Description of the Prior Art
While not so limited, the present invention is particularly applicable to measuring the leakage of current through or on the surface of insulating members of a component or a part of a electrical device particularly a circuit board. Surface insulation resistance testing, per se known in the art, is a test that yields data giving a measure of the cleanliness of the circuit board after wave solder and conformal coating and a wash process. Surface insulation resistance testing has become increasing important because the use of CFC (e.g. freon) to clean circuit boards has been prohibited to protect the environment. Residues that are both ionic and non-ionic have a direct impact on surface insulation resistance and long term reliability of a circuit board assembly.
There are numerous testing methods known to be established in various industries depending on the established criteria by users or others. Factors affecting insulation resistance measurements include temperature, humidity, residual charge, charging currents, time constant of the instrument and measuring circuit, test voltage, previous conditioning, and duration of uninterrupted test voltage application (sometimes called electrification time). The electrification time involves the lowering of current from an instantaneous high value to a steady state lower value at a rate dependent upon the factors such as test voltage, temperature, insulating materials, capacitance and external circuit resistance. The measurement of insulation resistance will increase for an appreciable time as test voltage is applied uninterruptedly. Therefore, it may take minutes to approach maximum insulation resistance readings but it is usually required that readings occur after a specified time of usually one or more minutes. The tests for the resistance of a dielectric surface are carried out by forming parallel lines of conductors bounding the dielectric circuit.
Test patterns may be created having different line spacings, line widths and lengths of conductor lines. Such test patterns are commonly referred to as a test coupon particularly when the test pattern is formed as an integral part of a circuit board and utilized during the fabrication of the circuit board to carry out the testing of a dielectric surface. To measure the resistance of surface insulation, a resistor on the test coupon comprised of a dielectric surface between parallel conductors is connected in a test circuit such that current flows from a voltage source through the resistor by applying the voltage source to one conductor and connecting the other conductor an input lead joined with ammeter or electrometer to measure the current. The current measurement is then utilized according to ohms law to calculate the insulation resistance, namely, the insulation resistance is equal to the applied voltage divided by the measured current across the resistor. When such a testing procedure is carried out manually, it is laborious task that is highly accessible to human error and electrical interference. Usually the test method is not reliable because it is not reproducible and highly operator dependent. Moreover, the exact location of the megohmmeter in the measuring circuit has an effect on the reading of the electrical current.
Other effects to the megohmmeter reading include electrostatic interference due to the temperature in the environment in which the test is carried out as well as the humidity and even a movement of personnel about the test site. When multiple samples must be tested, for example, automated testing is usually performed and requires a high voltage DC source which is supplied to one of the conductors for the resistor of each of the test coupons and an ammeter, megohmmeter or electro meter coupled to the second conductor of each of the resistor. Since multiple specimens are to be tested, the specimens are connected through a multiplexer relays to the meter. The output from the meter is supplied to a computer or microprocessor used to not only control the relays, but also display the test results.
Automated test systems have disadvantages that include cross-channel leakage which limits the test range, accuracy and repeatability of the tests. Many accepted test methods in industry require that the sample be biased at a predetermined DC voltage, for example 48 volts or 100 volts, for a predetermined period, usually a number of hours and then biased at a test voltage of 100 or 500 volts DC which is applied for a period of one or more minutes before the actual measurement of the insulation resistance is carried out. A serious deficiency of this automated test method occurs when the test voltage, for example 500 volts DC, is applied to all the channels of the sample at the same time. Cross-channel leakage and leakage of the relay open contacts results in a degrading of the accuracy of measurement to such an extent that reliable measurements can not be obtained.
Moreover, when the high voltage supply is exposed to a short circuit because a test coupon forms the short circuit or the coupon has such a low insulation resistance that it can short the high voltage supply, then the voltage across the remaining coupons is reduced to such an extent that it is impossible to measure with any accuracy the insulation resistance. To avoid this shortcoming, known available automatic test systems operate by applying a test voltage of some pre-determined level of direct current to only one sample for one or more minutes after which a measurement is made and after the measurement the test voltage is applied to the next sample for the specified period of time following which a measurement is made. Thus, the samples are tested sequentially, one at a time. This renders the time for testing the samples unduly excessive. For example, when the number of samples to undergo a test is 100 then the obtaining of a single set of data for all the samples will require at least 1.5 hours and up to 3 hours.
Moreover, when each test specimen undergoes numerous test cycles, the testing process is not only lengthy but very tedious requiring the accumulation of data in an orderly fashion in order to render the test meaningful and useful. In addition to the consumption of an inordinate amount of time to complete the test, the accuracy although carried out in a unitary test specimen fashion is subject to greatly varying errors. A test channel although an unused but adjacent to a test channel during a testing operation has a normal characteristic of the open relay of a resistance of 10.sup.10 ohms. This normal resistance of the unused open relay adds to the inaccuracy of the measurement to the insulation resistance which can vary from 10.sup.6 to 10.sup.12 ohms or more. The problem of inaccuracy is particularly acute when a test specimen takes the form of a four segmented comb pattern where the open relays of an adjacent coupon channel are charged electrically by the leakage current of a shorted coupon thus causing more inaccuracies to the measurement.
In a testing procedure for a circuit board, for example, it is common place to expose the circuit board to conditions which ages the materials through environmental conditions as well as electrical potential applied to test coupons. During the aging process, it is desirable to obtain measurements of the insulation resistance properties to the materials of the circuit board with known existing measuring methods and apparatus. Such a monitoring process of the materials would not be feasible because of the time required, for example, to complete a single set of test measurements where a large number of test specimens must be used. Thus, repetitive measurements of a large number of test specimens could not be completed in a sufficient time cycle to yield meaningful information about the aging process.
Accordingly, it is an object of the present invention to provide an improved method and apparatus for measuring electrical resistance of insulation wherein a low leakage current which is a function of surface insulation resistance is converted to a voltage in a circuit before a measurement is undertaken. A measurement signal is converted from a high source voltage (test voltage) and high source impedance (insulation resistance of specimen) to such a low magnitude of source voltage (output voltage of the converter e.g. an operational amplifier) and low source impedance (output impedance of the converter) so as to have no influence on the leakage from an open relay or switch contact that is used to multiplex the low voltage representing leakage current to the measuring device. The operational amplifier as a converter is chosen such that it has a very high input impedance and a very low output impedance.
A further object of the present invention is to provide a method and apparatus for measuring low leakage current from the surface of insulation wherein an applied test voltage to the insulation surface occurs through a serial arrangement of resistors between the high voltage supply and each of a plurality of test coupons, there being for each test coupon a discrete resistor to limit the current of a test voltage in the event a test coupon is shorted so that the measurement of the remainder of the test coupons remains unaffected by the shunt formed by the short.
Another object of the present invention to provide a method and apparatus for measuring low leakage current of surface insulation wherein the leakage current from an applied voltage from the insulation resistance is coupled to a converter wherein the leakage current is converted to a voltage by transformation from a high source impedance circuit i.e., the high voltage test circuit to a low source impedance circuit i.e., the output from the converter so that the low voltage, low impedance circuit operates with relay contacts therein in a manner to render leakage current due to open relay or analog switch negligible. The multiplexed output signal is advantageously fed to a measuring circuit to obtain insulation resistance value of the specimen under test.
It is a further object of the present invention to provide the method and apparatus for measuring the electrical resistance of insulation wherein a high test voltage is applied to a test specimen of such insulation for the purpose of measurement of leakage current and at other times while the test specimen is exposed to atmospheric conditions and/or applied voltages during which the test specimen may also be monitored for leakage current by the same circuitry utilized to monitor the leakage current of an applied high test voltage.